1. Field of Invention
This invention relates to a xenon lamp used as the light source in projectors using, for example, the Digital Light Processing (DLP®) technology.
2. Description of Related Art
In recent years, use of the digital projector, which enlarges and projects an image on a screen by optically illuminating picture elements, such as DMD (Digital Micro-mirror Device) and liquid crystal devices, using a light source with large output, has been increasing rapidly. High-intensity xenon lamps are used as the light source in such cases, and it is further required to increase the output to reduce the size.
FIG. 1 is a schematic cross sectional view showing a xenon lamp 1 as is disclosed, for example, in Japanese Patent Publication Number 2004-134104. The xenon lamp 1 is composed of: a discharge chamber 10, which is made of silica glass and provided with the light emitting part 2 and side tube parts 3a, 3b; a cathode 4 and an anode 5, which are arranged facing each other inside the light emitting part 2. The cathode 4 and the anode 5 are supported by a respective lead rod 6, which is made of tungsten. Moreover, a tubular supporting body 7, which is made of cylindrical silica glass, has through holes extending in an axial direction and is disposed and fixed within the side tube parts 3a, 3b. The lead rod 6 is sealed by a gradient binding part 8 in the side tube parts 3a, 3b extending through the tubular supporting body 7. The lead rods 6 project and extend outward from the outer end portion of the discharge chamber 10, and serve as the external leads by which electric power is supplied to each of the cathode 4 and the anode 5.
The intensity of the xenon lamp 1 is increased, to be specific, by adjusting the clearance between the cathode 4 and the anode 5 and the amount of the gas filling. For example, when the clearance between the cathode 4 and the anode 5 is shorter than that in a conventional xenon lamp and the amount of the gas filling is increased so that similar electrical properties as those of a conventional xenon lamp may be acquired, the electric input per unit arc length may increase and the outputted optical energy may become larger.
Since the light outputted from the xenon lamp 1 comprises not only the visible light range but the range from the ultraviolet region to the infrared region, if optical energy becomes larger, the output of ultraviolet light may also become larger.
The ultraviolet light of a wavelength of 200 nm or shorter among the light outputted from the xenon lamp 1 not only causes critical defects, such as the distortion of the discharge chamber 10, which is made of silica glass, but also generates ozone by reacting with the air existing around the xenon lamp. When ozone is generated, various damages may happen to optical instruments, such as a decrease in reflectance ratio of the collector mirror or the reflecting mirror and the transmittance of the filter; as a result, the illuminance in the irradiated area is reduced.
It is known that the ultraviolet light of a wavelength of 200 nm or less can be shielded by doping titanium oxide (commonly known as titania) on the discharge chamber 10, which constructs the xenon lamp, or creating a titanium oxide layer on the surface of the discharge chamber 10. By including titanium oxide in the discharge chamber 10, the ultraviolet light can be shielded and the generation of ozone can be prevented. The doping technique of titanium oxide is disclosed, for example, in Japanese Patent Publication Number H8-96751 (U.S. Pat. No. 5,608,227A), and the coating technique of titanium oxide is disclosed, for example, in Japanese Patent Publication Number H11-96970.
However, if the xenon lamp using silica glass and titanium oxide is turned on for a long time, white crystals are deposited on the surface of the glass that is exposed to the emission space of the discharge chamber. In the crystallized area, cracks have occurred on the surface of the emission space of the discharge chamber, and the titanium oxide layer, which shields the ultraviolet light, is destroyed. Therefore, the ultraviolet light generated in the emission space enters the cracks, critical defects, which cause the distortion of the silica glass that forms the discharge chamber, may happen, and this causes the burst of the discharge chamber.